1. Field of the Invention
The present invention relates to a semiconductor device manufactured by molding an element such as a semiconductor integrated circuit mounted on a lead frame with resin, and to a manufacturing method for the lead frame.
2. Description of the Related Art
A semiconductor device in which an element such as a semiconductor integrated circuit mounted on a lead frame is molded with resin is generally mounted onto a circuit board for use. In order to ensure sufficient mounting strength between the semiconductor device and the circuit board, leads of the semiconductor device are elongated to enlarge soldering area. Though solder wettability in a cut surface of leads of the semiconductor device, conventionally, has not been kept in good condition, large soldering area and Z-shaped forming of the leads make the solder wettability not only in both ends of the lead but also in heel portions of the leads good, permitting the solder to go up along the leads. Accordingly, a sufficient mounting strength can be ensured.
Large soldering area ensures mounting strength. However, along with the miniaturization of a semiconductor device, the mounting strength against a circuit board tends to be lower. Because increase in mounting density requires reduction both in lead area with which the semiconductor device is bonded to the circuit board, and in electrode area locating on a circuit board side. Solder is mainly used to mount a semiconductor device to a circuit board, and the mounting strength changes widely depending on whether or not the lead of the semiconductor device becomes wet easily with the solder. When the temperature of the solder reaches the melting point by reflow or the like, the solder melts together with plating covering the lead of the semiconductor device. At this time, formation of the plating on the entire lead surface of the semiconductor device is desirable, because a portion where plating is absent does not become wet by the solder and will have low strength. The smaller the semiconductor device is, the less the strength between the lead of the semiconductor device and the electrode of the circuit board becomes when bonded only with the solder. It is, thus, necessary to ensure an area which becomes wet with the solder as much as possible. In particular, the solder wettability is important in a lead-tip portion of the semiconductor device. This is because the lead-tip portion is apt to be affected by influence such as warpage of the circuit board when the semiconductor device is mounted onto the circuit board.
FIG. 6 is a schematic cross-sectional view showing a structure of a conventional semiconductor device. As shown in FIG. 6, in the conventional semiconductor device, a formed lead 2 protrudes from a resin 21. The lead 2 is covered by a plating layer 3 except a lead-tip end surface 12 in a lead-tip cut portion 10 made by cutting the lead 2. The presence of the lead-tip end surface 12 deteriorates the wettability to a soldering material, which serves as a bonding agent at the time of mounting onto a circuit board or the like.
FIG. 7 is a simplified side view showing a structure of a conventional semiconductor device in a manufacturing process step. As shown in FIG. 7, a lead 2, which is also a portion of a lead frame connecting two resins 21 and 31, has a uniform cross-section. When the lead frame having such a structure is cut and the semiconductor devices are separated from one another, the lead-tip end surface 12 as shown in FIG. 6 is formed.